The present invention relates to retroreflective sheeting employing microprism formations to retroreflect the light rays impinging thereon, and, more particularly, to such sheeting which is effective to provide retroreflection of light in a slightly divergent pattern to provide a concentration of the light within a relatively narrow viewing angle.
Retroreflective sheet material is widely employed for a variety of safety and decorative purposes, and is particularly useful when the need for night time visibility is significant under conditions of low ambient light. In retroreflective materials, the light rays impinging upon the front surface are reflected back towards the source of the illumination. In situations where headlights or search lights on boats and aircraft are the only source of illumination, this ability to retroreflect in a controlled cone the bulk of the rays falling thereon is especially significant for warning signs, delineators and the like.
Minnesota Mining and Manufacturing Corporation has manufactured retroreflective sheeting utilizing minute glass beads embedded in a matrix of synthetic resin to provide such retroreflection, and these materials have been sold under the trademark SCOTCHLITE. Illustrative of such materials is Bergeson et al. U.S. Letters Pat. No. 4,637,950 granted Jan. 20, 1987.
Applicant's assignee, Reflexite Corporation, has been marketing under the trademark REFLEXITE, reflective sheeting employing microprisms formations to produce such retroreflection. Illustrative of such materials is Rowland U.S. Letters Pat. No. 3,689,346 granted Sep. 5, 1972.
Among the applications for such retroreflective materials are reflective tapes and patches for clothing of firemen, reflective vests and belts, bands for posts and barrels, traffic cone collars, highway signs, warning reflectors, and the like.
Well made cube corner formations are known to be highly effective retroreflectors, but they tend to concentrate the reflected light in a very narrow zone directed to the originating light source. For highway and other applications, controlled divergence of the light rays within a narrow cone of 0.2.degree.-2.0.degree. is considered desirable to increase the angle over which the illuminated reflecting surface may be seen by drivers, pedestrians and the like who are displaced from the light source.
Efforts to increase the viewing angle are described in the prior art, and a review of some of the prior art is set forth in the initial portion of the specification of Hoopman U.S. Pat. No. 4,588,258, Hoopman describes the benefit of tilting the optical axes of adjacent prisms towards each other. Such tilting and the benefits thereof are also described in the earlier Heenan U.S. Pat. Nos. 3,541,606, and U.S. Pat. No. 3,923,378, Lindner U.S. Pat. No. 4,066,331, and White U.S. Pat. No 4,349,598.
Appledorn et al U.S. Pat. No. 4,775,219 describes improved divergence profiles obtained by repeating sub-arrays of prisms of distinctive shapes.
Stimson U.S. Pat. No. 1,671,086, U.S. Pat. No. 1,743,834 and U.S. Pat. No. 1,743,835 describe macro prism retroreflectors with faces which are arcuate to effect a controlled spread of the reflected light rays.
It is an object of the present invention to provide a novel microprism retroreflective sheeting which provides a desirable, controlled spread of the reflected light for good visibility over a 0.5.degree. viewing angle.
It is also an object of the present invention to provide such a retroreflective sheet material which may be readily fabricated and which has a high degree of brilliance.
Another object of the present invention is to provide methods for fabricating such retroreflective sheet material which are relatively simple and relatively economical, and which produce long-lived materials.